This invention provides an optical-wave circulator (OW circulator) that can be used indispensably in optical communication.
As is well known, a circulator, acting as a nonreciprocal element, has developed in microwaves, and so various types of the circulator are nowaday available. In the optical region, OW circulator which is adapted for the optical communication use has not been realized yet. There are technical difficulties in production of an OW circulator. One difficulty is that if we can apply the design technique of the microwave circulator to the production of the OW circulator, we have to make the OW circulator in the order of one tenth the wavelength of the light wave, or that long, as so we do in the microwaves. After the analogy of the microwave circulator, the OW circulator is to be made so delicately in the order of no less than 0.1 microns, (as the light wavelength for the optical communication is as short as from 1.0 to 1.5 microns) that machining of a magneto-optic structure (MO structure) is extremely difficult, and even if the MO structure is made in success, coupling of the input and output signal transmission lines with that very fine MO structure is actually another problem, since a glass-fiber transmission line, having the diameter of 5 microns for single-mode transmission and that of more than 50 microns for multiple-mode transmission, must be coupled with the MO structure having the dimensions of less than one tenth the wavelength in diameter. And furthermore, such a fine MO structure is exposed to the incident laser light beam that may have far more intensive power density possibly to give rise to nonlinear effect and to disturb normal performance.
The basic principle of the invention originates from a unique idea different from that of the microwave circulator. In embodiments of the invention, an MO structure having large dimensions than the light wavelength is utilized and incorporated in circulator embodiments to take advantage of various magneto-optic effects: Faraday effect, Cotton-Mouton effect, and magneto-optic Kerr effect. The Faraday effect is a phenomenon that the plane of polarization of the incident linearly polarized light, in passing through the MO material along the direction of magnetization (which is called Faraday location), is rotated due to the difference of refractive indices for left and right circularly polarized light waves: the Cotton-Mouton effect is a phenomenon of double refraction that the incident light wave, passing in the direction normal to the magnetization (which is called Voigt location), are distinctively retarded with variations of refractive indices for the light waves of polarizations normal and parallel to the magnetization; and the MO Kerr effect is a phenomenon of rotation of the polarization for the light waves reflected from the surface of the MO material.
The MO material is defined as such that the Faraday effect, the Cotton-Mouton effect, or the MO Kerr effect acts with biasing magnetic field. The most useful MO material is crystaline rare earth iron garnet that may have large Faraday rotation and small absorption losses, that is, large figures of merits (the ratio of the Faraday rotating angle and absorption losses for unit length).
These MO effects in the MO material can be explained in terms of tensor permittivity and also the MO anisotropic splitting factor under the biasing magnetic field, which presents a contrast to ferromagnetic material in microwaves that can be explained in terms of tensor permeability and its anisotropic splitting factor under the biasing magnetic field. The tensor permittivity is written by ##EQU1## and .epsilon.'/.epsilon..sub.1 is the MO anisotropic splitting factor. The ratio of .epsilon.'/.epsilon..sub.1 is far below the value of the ferromagnetic anisotropic splitting in the microwaves. The distinction between them is conspicuously essential. Underlying ideas in the invention are that light waves have shorter wavelength than the dimensions of the MO structure used, coupling between the light waves and MO material is not sufficiently strong, and therefore various MO effects should be turned to advantage.
The embodiments of the invention provide useful OW circulators for optical communication and optical integrated circuits. Features of the OW circulator embodiments are compactness, high efficiency and compatibility with the optical fiber transmission lines and optical IC techniques. More detailed explanation of the invention will be made below, referring to the drawings.